Extensive researches and developments are carried out based on the recent development of crystal-growth technologies. Specifically, in accordance with the recent semiconductor crystal technologies, it is possible to form a hetero-boundary structure in the order of several nanometers. As a result, the wavelength of intersubband transition in a quantum well is decreased, realizing intersubband transition in an optical communication wavelength band.
In addition, the optical signal processing is superior to the electric signal processing with respect to a transmission speed, intersignal-interference, and electric power consumption. Therefore, as an alternative to conventional switches using electric signal processing, there is a strong demand for a practical all-optical switch that utilizes only the optical signal processing.
Conventionally, operation of an all-optical switch using intersubband transition in a conduction band has been confirmed. This all-optical switch utilizes change in an absorption coefficient generated by interband and intersubband optical absorptions. Further, as an evolved type, an all-optical switch utilizing both of intersubband transition and interband transition in a conduction band of a quantum well structure has been studied (Non Patent Reference 1).
Recently, there are advanced researches on the luminescence of ultraviolet light utilizing interband transition between a conduction band and a valence band in high band-gap materials such as GaN and oxide materials. Therefore, by combining the intersubband transition in infrared light, it is possible to realize an all-optical switch utilizing an infrared controlling light and ultraviolet signal light. Thus, realization of all-optical switches in a broad wavelength band can be expected.
An optical device that utilizes intersubband transition in a semiconductor quantum well has superiority in response speed compared with semiconductor optical devices that utilize interband transition or the like. Currently, most of semiconductor optical devices which are mass produced mainly utilize absorption by interband transition. Response speed of an optical switch and optical modulator of higher speed is controlled in switching-off time by interband recombination time (in the order of several nano-seconds) of real excited carriers.
On the other hand, intersubband transition in the conduction band of a semiconductor quantum well structure, relaxation time is not longer than several pico seconds, thereby enabling enhancement of switching-off speed to one thousand or higher times higher than that in the case of interband transition.
However, III-V group semiconductors such as InGaAs, GaAs, GaN or the like are currently used in semiconductor lasers for optical communication and optical recording. To achieve an intersubband transition in a wavelength band of optical communication utilizing such materials as a quantum well layer, there are difficulties in crystal-growth technique.
For example, in a strained quantum well of InGaAs/AlAs or a strained quantum barrier (on InP substrate) that have been already reported, it is required to extremely narrow the width of a quantum well to the order of several atomic layers.
For example, FIG. 2 shows a single quantum well layer that has two quantum levels (subband: E1′, E2′). In order to shorten the intersubband transition wavelength using such a single quantum well layer, it is necessary to narrow the quantum-well width. In this case, E1′-E2′ intersubband transition has a well-width dependence as shown in FIG. 3. That is, intersubband energy increases and intersubband transition wavelength shortens in accordance with decreasing well-width.
In order to realize intersubband transition in a near infrared region, E2′ is required to be lower than the energy level of a barrier since both of quantum levels E1′ and E2′ increase with decreasing well-width. Therefore, in order to realize intersubband transition in 1.55 μm, a wavelength used in optical communication, a materials of very large interband discontinuity for enabling a very high barrier level are required, making it difficult to realize the intersubband transition.
Yoshida et al. proposed a method in which shortening of intersubband transition wavelength to the wavelength band of optical communication and high-speed, non-linear transition were realized consistently utilizing split levels of a coupled quantum well where two or more quantum wells were coupled. The intersubband wavelength was shortened to 1.55 μm by strengthening coupling between quantum wells in the coupled quantum well using an InGaAs well layer, an AlAsSb coupling barrier layer, and an AlAsSb outer barrier layer (see Patent Reference 1 and Non-Patent Reference 2).
On the other hand, where an optical switch is fabricated using an optical absorption by an intersubband transition of a quantum well, it is necessary to dope the quantum well with high concentration impurities such as Si in order to generate carriers. For example, in the materials using InGaAs well layer and AlAsSb outer barrier layer as shown in the below described Patent Reference 2 and Non-Patent Reference 1, mutual diffusion of constituent atoms of the quantum well was generated by high concentration doping of Si atoms, thereby disturbing the flatness of hetero-boundaries. Proposed solutions for such problems include improvement of flatness of hetero-boundaries by inserting AlAs diffusion inhibiting layer between the InGaAs well layer and the AlAsSb outer barrier layer, and an use of AlAs layer as a coupling barrier layer alternative to the AlAsSb layer. As a result, a satisfactory absorption by intersubband transition is observed (see Patent Reference 2 and Non-Patent Reference 3).
However, in the above-described coupled quantum well structure (FIG. 4), in order to shorten the intersubband transition wavelength to 1.55 μm by strengthening the coupling between quantum wells, it was required to decrease the AlAsSb or AlAs coupling barrier layer to 4 atomic layers and to decrease the InGaAs well layer to 2 nm. Due to the constraint on semiconductor crystal growth technology, film thickness of constituent layers of a quantum well cannot be decreased over a lower limit. In addition, considering the yield of mass production, further reduction of film thickness of the coupling barrier layer, well layer or the like are not desirable. To control more broad wavelength of intersubband transition, it is necessary to further strengthen the coupling between quantum wells.
On the other hand, Tsuchida et al. recently found an ultra-high speed all-optical phase modulation effect. Where the electrons in a subband are excited by irradiating pump light of TM polarization wave to an optical waveguide that uses an InGaAs/AlAsSb coupled quantum well structure as a core layer, modulation of phase occurs in a probe light of TE polarization wave in which a subband absorption does not occur. By a combination of this phase modulation effect and a Mach-Zehnder interferometer, all-optical DEMUX operation of 160 GHz was realized with a low energy optical input of 7pJ. For further reduction of operation energy, it is effective to enhance the phase modulation effect (see Non-Patent References 4 and 5).
On the other hand, Ishikawa et al. made clear that the above-described phase modulation effect was caused by a huge change in a refractive index caused by aparabolic (non-parabolic) E-k relation in the quantum well and 2 dimension plasma effect of high concentration carriers. Based on this result, it is estimated that a strengthening of the coupling between quantum wells is an effective way to enhance a phase modulation effect (see Non-Patent References 6 and 7).    Non-Patent Reference 1: Naruse et al., IEEE Photon. Technol. Lett., 17, (2005) 1701.    Non-Patent Reference 2: Yoshida et al., IEICE Tans. Electron., E87-C, (2004) 1134-1141.    Non-Patent Reference 3: Mozume et al., Jpn. J. Appl. Phys., 42 (2003) 5500.    Non-Patent Reference 4: Tsuchida et al., Opt. Lett., 32 (2007) 751.    Non-Patent Reference 5: Akimoto et al., Proceeding of 2007 International Conference on ECOC, to be published.    Non-Patent Reference 6: Ishikawa et al., Jpn. J. Appl. Phys., vol. 46, pp. L157-L160, 2007.    Non-Patent Reference 7: Nagase et al., Proceedings of 2007 International Conference on IPRM, 2007 p. 502.    Patent Reference 1: Japanese Unexamined Patent Application, First Publication, No. 2000-89270.    Patent Reference 2: Japanese Unexamined Patent Application, First Publication, No. 2003-329988.